It's important to realize that resonance mesomers do not exist! Resonance is just a convenient "back of the envelope" way to calculate the properties of the frontier molecular orbitals of a conjugated system without solving any equations. For example, 'resonance' shows that an enolate is nucleophilic at both the oxygen and the alpha-carbon, but the real reason for this is that the HOMO has large lobes on these two atoms (i.e. a great deal of the negative charge lies on them) - resonance tells us the answer, but doesnt explain the reason why it is so.
You also ask why a compound with many resonance structures is more stable than those with a low number. Again resonance tells us the answer, but not the reason why. In order to understand this, you would need to take a molecular orbital, and try 'mixing' it with an additional pi bond or p-orbital - the interaction between occupied and unoccupied orbitals leads to a net lowering of the energy of the occupied MOs, hence the lower energy.
Regarding your query about ortho/para directing groups, the same argument applies as for the enolate above - the HOMO has large lobes at these positions due to the interaction of the aromatic pi system with the high energy electrons (e.g. lone pair) of the directing group. There is a great deal of evidence for this, including the ortho/para directing effect itself, as well as NMR chemical shift data, and computation. There are probably loads more, just not off the top of my head.
Hope this helps